Up to now, refrigerators such as those having a compression-refrigerating cycle of a compressor, a condenser, an expansion valve, and an evaporator use CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) as their refrigerants. In addition, many kinds of lubricating oil have been produced and employed in combination with such refrigerants. However, concerns are that the CFC compounds, which have been conventionally used as refrigerants, may destroy the ozone layer when the CFC compounds are discharged into the atmosphere and cause environmental pollution problems. In recent years, for measures against the environmental pollution, HFCs (hydrofluorocarbons), which may be alternatives for the CFC compounds, have been developed. A variety of so-called CFC substitutes including 1,1,1,2-tetrafluoroethane (R-134a) with a little fear of environmental pollution have become commercially available. However, concerns arise that the above-mentioned HFCs also cause environmental pollution problems. Thus, use of natural refrigerants without such problems and the like have been considered.
On the other hand, studies on carbon dioxide (CO2), ammonia, and hydrocarbon gas have been made as natural refrigerants which substantially do not contribute to destruction of the ozone layer and global warming and will be provided as refrigerants in near feature.
For example, carbon dioxide (CO2) is harmless for the environment and excellent from the viewpoint of safety for human, as well as having advantages of, for example, (i) its pressure almost at the optimal economical level; (ii) an extremely small pressure ratio, compared with that of the conventional refrigerant; (iii) an excellent adaptability to normal oil and structural materials of a machine; (iv) being available all over the place without any difficulty; and (v) extremely cheap price without the need of recovery. In addition, carbon dioxide (CO2) has been used as refrigerants for some of the conventional refrigerators and the applications thereof as refrigerants for car air conditioners and heat pumps for hot water have been investigated in recent years.
Typically, for example, a compression refrigerator contains at least a compressor, a condenser, an expansion mechanism (e.g., an expansion valve), and an evaporator. Such lubricating oil for a compression refrigerator has a structure in which a liquid mixture of refrigerator oil, i.e., lubricating oil for refrigerant compressors and a refrigerant circulates in this closed system. In the compression refrigerator, although it depends on the kind of the apparatus, the inside of the compressor reaches a high temperature and the inside of the refrigerating chamber reaches a low temperature in general. Thus both the refrigerant and the lubricating oil should circulate in the system without causing phase separation within a wide temperature range from low to high temperatures.
In general, a temperature region in which the refrigerant and the lubricating oil are compatible, i.e., not phase-separated, is preferably in the ranges of −20° C. or less and 0° C. or more, more preferably in the range of 10° C. or more on the higher temperature range. If the phase separation occurs in the refrigerator at work, it will have a significantly adverse effect on the life or efficiency of the apparatus. For example, when the phase separation of the refrigerant and the lubricating oil occurs at a compressor part, it leads to insufficient lubrication in a moving part and causes burn out or the like, thereby significantly shortening the life of the apparatus. On the other hand, when the phase separation occurs in the evaporator, it leads to a decrease in heat exchange efficiency due to the presence of high viscous lubricating oil. The lubricating oil for a compression refrigerator is employed for lubricating the moving part of the refrigerator, so that its lubrication property is obviously considered to be also important.
In particular, the inside of the compressor becomes a high temperature, so it can be important for the lubricating oil to have a viscosity enough to retain an oil film to be required for lubrication. The required viscosity of lubricating oil varies depending on the kind of the compressor to be used and the use conditions thereof. In general, however, the viscosity (kinetic viscosity) of lubricating oil yet to be mixed with the refrigerant is preferably 1 to 50 mm2/s, particularly preferably 5 to 20 mm2/s at 100° C. If the viscosity is lower than the defined value, a resulting oil film is thin and tends to cause insufficient lubrication. In contrast, if the viscosity is higher than the defined value, the heat exchange efficiency may be reduced. On the other hand, like a car air-conditioner, when it is designed for use in cold regions, the viscosity of lubricating oil should not be too high at low temperatures to ensure its ability of allowing the apparatus to be initiated. Therefore, the lubricating oil requires a lower pour point and a higher viscosity index. In general, the lubricating oil is required to have a pour point of −20° C., preferably −30° C. or less, more preferably −40° C. or less and a viscosity index of at least 80 or more, preferably 100 or more, more preferably 120 or more.
Further, the refrigerator oil requires various characteristics including lubricity and hydrolytic stability, as well as refrigerant compatibility and low-temperature fluidity. However, the characteristics of the refrigerator oil are easily affected by the kind of the refrigerant. When the refrigerator oil for a fluorocarbon refrigerant, which has been commonly used up to now is employed together with a natural refrigerant such as a carbon dioxide refrigerant, it is difficult to satisfy many characteristics that are required.
The development of novel refrigerator oil suitable for use with natural refrigerants, in particular, carbon-dioxide refrigerants, has been progressed. Polyalkylene glycol (PAG) has comparatively high compatibility to the carbon-dioxide refrigerant and is also excellent in low-temperature fluidity and hydrolytic stability, so it has drawn attention as one of substrates of refrigerator oil for carbon-dioxide refrigerants (see, for example, Patent Document 1).
The conventional PAG refrigerator oil described above shows compatibility to the carbon-dioxide refrigerator in a composition with a low proportion of the carbon-dioxide refrigerant, but the range of compatibility is not always sufficient. Therefore, there is a method for preparing PAG with low viscosity to provide such refrigerator oil with sufficient refrigerant compatibility. In this case, however, it tends to fall in a vicious cycle of being insufficient in lubricity and stability.    Patent Document 1: JP 10-46169 A